In various fields such as buildings, bridges, ships, offshore structures, construction machinery, tanks, and penstocks, steel materials are welded in accordance with shapes of steel structures to form desired shapes. In recent years there has been remarkable development in the production of larger scale steel structures, and thus there has been significant progress toward higher strength and thicker steel materials used to produce such steel structures.
However, when attempting to produce a steel plate having a thickness of 100 mm or greater and also having excellent strength and toughness in a mid-thickness part thereof, the large thickness of the steel plate causes the thickness central part to experience a lower cooling rate, which facilitates formation of a microstructure such as ferrite that has relatively low strength. Consequently, it is necessary to add large amounts of alloying elements to inhibit formation of such a microstructure.
It is particularly important to form a bainite microstructure or a mixed microstructure of bainite and martensite in the mid-thickness part during quenching to improve strength and toughness of a mid-thickness part of a steel plate. Accordingly, it is necessary to add large amounts of alloying elements such as Mn, Ni, Cr, and Mo.
Publications related to such steel plates include Nippon Steel Technical Report No. 348 (1993), p. 10-16 and NKK Corporation Technical Review No. 107 (1985), p. 21-30. Nippon Steel Technical Report No. 348 (1993), p. 10-16 describes a steel plate having a plate thickness of 210 mm and NKK Corporation Technical Review No. 107 (1985), p. 21-30 describes a steel plate having a plate thickness of 180 mm.
However, when large amounts of alloying elements such as Mn, Ni, Cr, and Mo are added to improve the microstructure of a mid-thickness part as described above, there is a problem that even if heat treatment is carried out with an objective of refining and homogenizing prior γ grain size, the desired refinement of prior γ grain size may not occur and, as a result, it may not be possible to obtain adequate toughness in the mid-thickness part.
We believe that the phenomenon described above occurs due to a shear-type reverse transformation. Specifically, nucleation and growth of γ grains normally occur from prior γ grain boundaries during heating of a steel material, and refinement and homogenization of prior γ grain size occur in association therewith. However, in a situation in which large amounts of alloying elements are contained in the steel material, nucleation and growth of γ grains are less likely to occur as described above and a shear-type reverse transformation may occur in which the prior γ grains themselves undergo a sudden reverse transformation to austenite. Consequently, γ grains remain coarse in a part of the steel material in which this reverse transformation occurs. Moreover, bainite and martensite obtained by cooling from this state are also coarse.
However, Nippon Steel Technical Report No. 348 (1993), p. 10-16 and NKK Corporation Technical Review No. 107 (1985), p. 21-30 do not describe a technique that resolves the difficulty of refining prior γ grain size during heat treatment. Therefore, a need remains to reliably produce steel plates having excellent strength and toughness in a mid-thickness part thereof.
It could therefore be helpful to provide a steel plate having excellent strength and toughness in a mid-thickness part thereof, despite having a plate thickness of 100 mm or greater, and to provide a method of producing such a steel plate.